Wheat is the most widely cultivated cereal and is one of the staple foods for the world's population. It is used to produce wheat flour that is used in the composition of bread, pasta, cereals and many other food products intended for man or animals. It is known that wheat, like all other cereals, is rich in starch and that a change in the quantity or quality of starch affects the flour derived from this wheat.
The flour searched for is flour derived from starch with physical properties such that products containing these flours do not retrograde or become stale.
The starch is in the form of simple or compound insoluble grains or granules with a spherical, lenticular or angular shape and with variable size depending on the species. The composition of a starch granule is complex and different in different species. Firstly it comprises 15 to 20% of water, but also fatty acids (1% in wheat), calcium phosphate (cereals) and potassium phosphate (potatoes) forming part of a mineral residue that can be as much as 0.4% of the fresh weight (wheat starch), and finally proteins, most of which are enzymatic.
Starch in the native state is completely insoluble. When treated in hot water (from 60° C. to 80° C.), starch grains that are insoluble in cold water produce a paste, that is a viscquous liquid that hardens as it dries. Therefore, they contain amylopectin, a mucilaginous substance that causes the viscosity of the paste, and amylose that is precipitated after cooling in the form of a white powder.
Therefore, this heat/moisture treatment causes irreversible swelling of the granules and their solubilization. It is necessary so that the properties, and particularly viscoelasticity, polymerization and retrogradation, can be revealed.
The heat/moisture treatment causes an irreversible modification of the semi-crystalline structure of native starch, which changes from an ordered state to a disordered state, resulting in gelatinization. The gel thus formed is not stable and after a few days the starch restructures itself into a more ordered structure, this phenomenon being known as retrogradation. The fact that bread products become stale and the excessive viscosity of sauces and other products of this type are related to this retrogradation and to another phenomenon called syneresis (exudation of water) responsible for separation of phases and decomposition of the texture of products.
These problems do not arise in traditional cooking, when food is eaten quickly. In this case, unmodified wheat or maize flours are used to thicken a sauce or soup, etc. But problems due to retrogradation occur as soon as it is required to conserve these products, particularly by freezing. The texture of the food can be recovered to a certain extent, but the result is never satisfactory.
These problems occur on a larger scale in the food industry. In that context, food is not intended for immediate consumption. It is essential that it should be conserved as well as possible for as long as possible.
These problems limit the use of starches with a high amylose content, and are one of the reasons for which waxy starches are required.
Some species, and particularly maize, are already known that contain starches composed exclusively of amylopectin derived from waxy mutants. These mutants are in strong demand because they have a limited tendency towards retrogradation, thus guaranteeing a texture that does not change with time.
Waxy mutants are plants in which the synthesis of Granule Bound Starch Synthase (GBSS), also called Wx protein, an essential enzyme for the synthesis of amylose, is inhibited, or plants that synthesize inactive GBSS. Most waxy starches are derived from diploid plants such as barley, maize, rice or potatoes.
The techniques used to obtain waxy starch are more adapted to diploid plants. For example, mutageneous treatment using X-rays or ethyl-methane-sulphonate are very suitable for diploid plants, but are inefficient on tetra or hexaploid plants which do not express recessive mutation. Therefore, it is more difficult to obtain waxy mutants starting from polyploid plants considering the presence of several isoenzymes that take part in the synthesis of amylose.
Thus, wheat is polyploid and most wheat varieties are hexaploid, with three groups of seven pairs of chromosomes. There are three coding genes for three isoforms of GBSS, Wx-A1, Wx-B1 and Wx-D1. These three isoforms of GBSS may be separated as a function of their molecular weight. Furthermore, they have different structural, chemical and biochemical properties.
A few varieties of natural “partial waxy” wheat have been identified in which one or two of the iso-enzymes are inactive or missing. Starches derived from this wheat are also called “partial waxy”. The concentration in amylose in these “partial waxy” starches is low and variable, but it is not zero, and in some cases the content of amylose remains almost unchanged by a compensation phenomenon.
On the other hand, the existence of a natural full waxy mutant is very improbable, since a recessive mutation would be necessary on each of the three loci.
One selection method was developed and described to obtain full waxy hexaploid wheat (Toshiki Nakamura et al., Mol Gen Genet, 1995, No. 248, p. 253-259). It consists of crossing a variety of partial waxy hexaploid wheat mutated on the A and B genes, Kanto 107, and a “partial waxy” hexaploid wheat variety mutated on the D gene, Bai Huo. A small percentage of seeds derived from this cross are full waxy and generations originating from these seeds have the full waxy phenotype.
The method used to obtain products according to this invention also consists of crossing two varieties of hexaploid wheat, one being an aabbDD double mutant and the other mutant on the D genome, AABBdd but adapted to western farming. The main varieties mutated on one of the genes involved belong to varieties not adapted to western farming, either because they are very old and very large varieties with a low potential yield, or because they are varieties from other countries that adapt poorly to western conditions. There are several aabbDD type varieties in the West, for example IKE and ROSELLA.
It would be expected that products originating from full waxy wheat would have an undesirably low viscosity and elastic texture. This preconception is perpetuated and disseminated particularly in Canadian patent application No. 2 194 944 in which waxy wheat flours are only used mixed with traditional flours to obtain a satisfactory product.